1. Field of the Invention
The present invention relates to a reversible fluid pump that can work with a constant intake or delivery rate and to a hydraulic system using such a pump.
It more particularly relates to a reversible pump with two alternating pumping elements interconnected in series or in parallel, that can work with a constant intake or delivery rate in a fluid circulation system such as those that can be found in various industry types. The pump according to the invention finds applications notably in chromatography systems.
2. Description of the Prior Art
Various pump types can be used for circulating liquid mixtures such as, for example, alternating pumps comprising generally two alternating linear pumping units arranged in parallel.
According to the embodiment of FIG. 1, each of the two units arranged in parallel comprises a piston, sliding in a cylinder 2 communicating, by means of a one-way valve 4 opening during the intake phase, with an inlet line 3 coming from a first tee T1 delivering a liquid L. The two units PU1, PU2 also communicate, through an outlet line 5 and by means of valves 6 opening during the delivery phase, with a second delivery tee T2. Phase shift of units PU1, PU2 is controlled by power sources M under the control of a control device PC so that the intake phase of one substantially corresponds to the delivery phase of the other.
Various types of power sources M can be used.
The stroke depth of each rod 1 in its cylinder 2 can for example be provided (FIG. 3) through the translation of a screw 7 resting on the head of piston 1 by means of a ball thrust 8. The screw translation comprises for example a nut 9 threaded to screw 7, that is for example housed in the hollow rotor of a stationary electric motor 10 and driven in rotation thereby. The direction of translation of the screw is changed by inverting the direction of rotation of the motor every pumping half cycle.
According to a second embodiment (FIG. 4), the stroke depth of piston 1 in body 2 is provided by the rotation of a cam 11 resting against the head of piston 1, whose pin 12 is driven in rotation by a motor 13. The stroke depth of piston 1 in the inner cavity of body 2 is obtained by changing the offset .DELTA. of the cam on the axis thereof. In either case, motor 13 is driven by a control computer PC.
The speed of each piston 1 decreases at the end of its stroke, and consequently so does the flow delivered thereby. If the global rate of delivery of units PU1, PU2 (FIG. 1) has to be substantially constant, the sum of the speeds of the two pistons must remain constant and the delivery phase of unit PU2 for example must therefore start before first unit PU1 has totally finished. During the relatively short fraction of each cycle where the two units deliver at the same time, the intake rate is zero, which results in a pulsed intake rate of the pump.
In a pump with two units PU1, PU2 arranged in series (FIG. 2), unit PU1 draws fluid out of a fluid tank R through a line TE on which a first nonreturn valve V1 is interposed. It drives it through a second nonreturn valve V2, towards unit PU2. The latter drives the pumped fluid through a line TS to the pump outlet. When piston 1 of unit PU1 is in the delivery phase and drives a volume .DELTA.V towards unit PU2 downstream, piston 1 of unit PU2 is moved back so that it draws a volume .DELTA.V.sub.s =.DELTA.V/2 taken from the volume delivered by first unit PU1. The volume expelled through line TS is thus equal to .DELTA.V/2. When piston 1 of the same unit PU1 goes into the intake phase and valve V2 closes, piston 1 of second unit PU2 goes into the delivery phase and expels the previously drawn volume .DELTA.V.sub.s =.DELTA.V/2 towards line TS. The flow expelled through line TS is constant provided that the motion laws applied to both pistons are so selected that the sum of their respective speeds is permanently constant. An example of a pump of this type is described in U.S. Pat. No. 5,755,561 filed by the assignee.